so when a FLAC comes along, it usually blows me ears into my brain.
FLAC files do not contain any gain field to be adjusted in this way. The entire file must be decoded, scaled, then re-encoded, and the process is not lossless, since there may be minor quantization errors.
AFAIK It's not possible to change the target level for foobar2000 RG scanner.
Note that the clipping issue you mentioned is not possible with FLAC files, since they only support integer samples, and those can't clip.
The discussion seems to be getting sidetracked; the OP was asking about MP3s to which he applied RG.
MP3 files weren't even mentioned in the first post.